Conventionally there is proposed an illuminating light communication device which is adapted for a lighting fixture having a light-emitting diode(s) (an LED(s)) as a light source and configured to modulate an intensity of an illumination light to transmit a signal (see Japanese Patent Application Publication Number 2012-69505 (hereinafter referred to as “Document 1”) for example). In the illuminating light communication device, no special device such as an infrared communication device is required, because the illumination light per se is modulated and thereby the signal is transmitted. In addition, electric power can be saved by employing the light-emitting diode(s) as a lighting source, and accordingly the usage thereof for a ubiquitous information system in underground malls has been studied.
The illuminating light communication device described in Document 1 includes a constant current source, a smoothing capacitor, a load circuit, a load change element, a signal generation circuit and a switch element. The smoothing capacitor is adapted to reduce the ripple of an output of the constant current source. The load circuit includes light-emitting diodes and is supplied with the output of the constant current source. The load change element is formed of a resistor connected in parallel with a part of the light-emitting diodes, and adapted to be added to the load circuit, thereby partially changing a load characteristic (an impedance characteristic) of the load circuit. The signal generation circuit is configured to generate a binary optical communication signal. The switch element is formed of a switching device connected in series with the resistor as the load change element, and turned on and off in accordance with the optical communication signal, thereby switching whether to add the load change element to the load circuit or not. As a result, a load current flowing through the light-emitting diodes is modulated into a current having waveform corresponding to the optical communication signal, because the load characteristic of the load circuit varies according to the optical communication signal.
There is however a problem in the prior art that a modulation rate (or factor) cannot be changed easily, because the resistor as the load change element consists of a fixed resistor. For example, when a lighting system is installed in a new building, a design change of illumination thereof may be made during the installation of the lighting system. In this case, if the prior art is employed as the lighting system, the resistor needs to be exchanged in accordance with a required modulation rate. As a result, the modulation rate cannot be easily changed.